I thought my DS was a little young for this, he's almost 16 months old.

He has a history of UTI's (5) so i was a little concerned that he wasn't peeing as much, but his recent culture came back totally clean (yesterday). Yesterday i changed him when he got up at 7:30, and he was still dry at 11! Around 11:30 he finally peed and it was a huge pee. Same thing today, he got up at 8am and i changed him and he didn't pee until around 11. Is this normal? They did not that his urine was SLIGHTLY concentrated but nothing major, and he drinks water all day and nurses a few times a day.

So is this normal? Is he just learning to hold it? (I'm pretty sure he knows how to hold it, as we've NEVER been able to get a urine sample in the baggie from him. No matter how long it's been since he's peed, he'll never pee with the baggie on...but when they cath him his bladder will be full!)

That's normal. They do fewer dribble pees & do more full pees. That helps with PLing since they aren't "spotting" in their pants every 20 minutes. He probably is just developing his natural rhythm.
Now he'll go from dry to soaked in 15 seconds flat!

maybe try sitting him on the potty at 10:55 and see if he goes , my son was the same but a little later , I found he had a dry diaper at night untill 8 then would soak the begeezez out of it , just started to put hiom on the potty ,1 less diaper at a time ,